1. Field of the Invention
This invention relates to amplitude adjustment devices such as amplitude compression devices and amplitude expansion devices, which are made with the MOS (i.e., Metal-Oxide Semiconductor) technology. In addition, this invention also relates to full-wave rectifiers, applicable to the amplitude adjustment devices, which are made with the MOS technology. Specifically, the devices are used for amplitude adjustment and rectification of audio inputs of digital audio systems.
This application is based on Patent Application No. Hei 10-180864 and Patent Application No. Hei 10-180865 both filed in Japan, the contents of which are incorporated herein by reference.
2. Description of the Related Art
Conventionally, amplitude compression/expansion devices are used for compressing and expanding signals of audio playback systems or audio reproduction systems. In the case of the automobiles, for example, drivers normally hear the noise due to the running of the automobiles when listening to the music which is played back with audio devices. So, if the drivers play back the music having a broad dynamic range such as the classic music, the drivers are hard to listen to piano sounds which are performed in pianissimo, for example. To improve such hardness in listening to the music in the automobiles, amplitude compression devices called xe2x80x9ccompressorsxe2x80x9d are used for the audio devices so that musical tone signals having small amplitudes are reproduced with a relatively large gain while musical tone signals having large amplitudes are reproduced with a relatively small gain.
There are provided three examples as the aforementioned amplitude compression devices, as follows:
FIG. 11 shows a circuit configuration for a first example of the amplitude compression device, which uses a voltage control amplifier. Herein, the voltage control amplifier 100 contains a multiplier, which is configured using bipolar transistors. The voltage control amplifier 100 adjusts an amplitude of an input signal Vin based on a control signal Cs. Thus, the voltage control amplifier 100 produces an output signal Vout in response to the input signal Vin. An amplitude detection circuit 110 is configured by a full-wave rectifier and a low-pass filter. The amplitude detection circuit 110 produces the control signal Cs in response to an amplitude of the output signal Vout. Normally, the bipolar transistors have base-emitter voltage characteristics, which show logarithmic characteristics. Using such characteristics, the voltage control amplifier 100 adjusts the amplitude of the input signal Vin.
FIG. 12 shows a circuit configuration for a second example of the amplitude compression device, which uses a gain switching amplifier. Herein, the gain switching amplifier 200 has a capability of switching over gains thereof based on control data Dc. In addition, an amplitude detection circuit 210 detects an amplitude of an output signal Vout. So, the amplitude detection circuit 210 produces the control data Dc in response to the detected amplitude. Incidentally, the gain switching amplifier 200 has a number of steps in changing the gains, which are called xe2x80x9cgain stepsxe2x80x9d. Herein, the number of gain steps corresponds to a number of bits of the control data Dc.
FIG. 13 shows a circuit configuration for a third example of the amplitude compression device, which uses a digital signal processor (i.e., DSP). Herein, an input signal Vin is supplied to a DSP 310 via an analog-to-digital converter (or A/D converter). The DSP 310 detects an amplitude of the input signal Vin. Then, the DSP 310 performs non-linear amplification based on the detected amplitude, thus producing output data thereof A digital-to-analog converter 320 (or D/A converter) converts the output data of the DSP 310 to an analog signal, which is output as an output signal Vout.
The aforementioned examples of the amplitude compression devices suffer from problems, as follows:
The first example of the amplitude compression device shown in FIG. 11 is designed such that the voltage control amplifier 100 is configured using the bipolar transistors, wherein amplitude compression is performed using the logarithmic characteristics of the bipolar transistors. So, it is impossible to manufacture the amplitude compression device in a form of an IC in accordance with the MOS process (or MOS technology). For this reason, the first example of the amplitude compression device suffers from a problem in which it has a limited range of application.
In the second example of the amplitude compression device, the gain switching amplifier 200 cannot change the gains thereof in a continuous manner. Therefore, the output signal should be made discontinuous in response to gain switching timings. Thus, the second example suffers from a problem in which it cannot produce the output signal which is xe2x80x9csmoothxe2x80x9d.
The third example of the amplitude compression device uses the DSP 310, which requires conversion from analog signals to digital signals and conversion from digital signals to analog signals. For this reason, the third example suffers from a problem in which it has a complicated circuit configuration.
By the way, full-wave rectifiers are known as devices that perform full-wave rectification on signal voltages to detect amplitude values of signals. FIG. 14 shows an example of a circuit configuration for the full-wave rectifier. The full-wave rectifier of FIG. 14 is mainly configured by a half-wave rectifier and an addition circuit of an inversion type. Herein, the half-wave rectifier is configured by resistors 110, 120, diodes D1, D2 and an operational amplifier OP1, while the addition circuit is configured by resistors 130, 140, 150 and an operational amplifier OP2. All of the resistors 110 to 140 have same resistance xe2x80x9cRxe2x80x9d, while the resistor 150 has resistance of xe2x80x9cR/2xe2x80x9d.
The half-wave rectifier is configured such that the diodes D1, D2 cancel voltage drops Vf in forward directions. Therefore, a half-wave rectified signal Vxe2x80x2 increases in a positive direction from a ground level. For example, if an input signal Vin shown in FIG. 15A is applied to the half-wave rectifier, its half-wave rectified signal V is shown in FIG. 15B.
In the addition circuit of the inversion type which is configured by the resistors 130 to 150 having the aforementioned resistances respectively, it is possible to perform addition on the input signal Vin with a gain xe2x80x9cxe2x88x921xe2x80x9d, while it is possible to perform addition on the half-wave rectified signal Vxe2x80x2 with a gain xe2x80x9cxe2x88x922xe2x80x9d. Therefore, an output signal Vout of the addition circuit is shown in FIG. 15C.
As described above, the full-wave rectifier is configured using two diodes and two operational amplifiers (OP1, OP2), wherein the half-wave rectified signal Vxe2x80x2 is produced and is mixed with the input signal Vin so that the output signal Vout is created.
The aforementioned full-wave rectifier can be applied to an audio signal processing circuit in order to detect amplitudes of reproduced audio signals, wherein processing is performed in response to the amplitudes of the reproduced audio signals. Engineers wish to manufacture such audio signal processing circuit in a form of a LSI circuit in accordance with the CMOS process (where xe2x80x9cCMOSxe2x80x9d is an abbreviation for xe2x80x9cComplementary Metal-Oxide Semiconductorxe2x80x9d). However, it is impossible to form the diodes by the CMOS process. So, there is a disadvantage in that the diodes should be provided as external components which are attached to the LSI circuit.
It is an object of the invention to provide an amplitude compression device and an amplitude expansion device, which have simple circuit configurations and which can be manufactured as ICs in accordance with the MOS process with ease.
It is another object of the invention to provide the amplitude compression device and amplitude expansion device, in which gains can be varied continuously.
It is a further object of the invention to provide a full-wave rectifier, which is configured using field-effect transistors without using diodes being externally connected.
In one aspect of the invention, there is provided an amplitude adjustment device such as an amplitude compression device and amplitude expansion device, which is basically configured by a PWM modulator, a demodulator and an amplitude detector. Herein, the PWM modulator effects pulse-width modulation on an input signal to produce a pulse-width modulated signal, which is demodulated by the demodulator to produce an output signal (and a demodulated signal). In addition, the amplitude detector detects an amplitude of the demodulated signal or an amplitude of the input signal to produce a control signal. A modulation factor of the pulse-width modulation is adjusted based on the control signal. Herein, the control signal controls a feedback value, which corresponds to a fraction of the pulse-width modulated signal and which is fed back through a negative feedback loop in the PWM modulator. In the case of the amplitude compression device, for example, an input/output gain is changed inversely proportional to the amplitude of the input signal or amplitude of the output signal. That is, the input/output gain is increased as the amplitude of the input signal (or output signal) decreases, while the input/output gain is decreased as the amplitude of the input signal (or output signal) increases. Thus, it is possible to compress a dynamic range with respect to input/output characteristics.
In another aspect of the invention, there is provided a full-wave rectifier, applicable to the amplitude adjustment device, which is mainly configured by an inversion amplifier, an amplifier and an output section. Herein, the inversion amplifier amplifies an input signal with a gain of xe2x80x9cxe2x88x921xe2x80x9d, while the amplifier amplifies it with a gain of xe2x80x9c1xe2x80x9d. Outputs of the amplifiers differ from each other in phases by 180xc2x0. The output section produces a full-wave rectified signal based on the outputs of the amplifiers. Specifically, the output section selects either the output signal of the inversion amplifier or the output signal of the amplifier in response to every half of one period of the input signal. For example, the output signal of the inversion amplifier is selected and is used for formation of a first portion of a full-wave rectified signal in a first half duration of one period of the input signal. In addition, the output signal of the amplifier is selected and is used for formation of a second portion of the full-wave rectified signal in a second half duration. The first and second portions are combined together to form a xe2x80x9cnegativexe2x80x9d waveform for the full-wave rectified signal in response to one period of the input signal. Incidentally, all of the amplifiers and output section are configured using field-effect transistors without using diodes being externally connected. Hence, it is possible to manufacture the full-wave rectifier in a form of an IC in accordance with the MOS process with ease.